1. Field of the Invention
The present invention relates to a light-receiving circuit applicable to an optical receiver and an optical transceiver each constituting a portion of an optical network
2. Related Prior Art
In the conventional optical receiver and the optical transceiver, light-receiving circuits are widely used in which an avalanche photodiode (hereinafter denoted as APD) is installed as a light-receiving device.
Japanese Patent publication JP-2000-244418A has disclosed a light receiving circuit that provides a PIN-PD for receiving light with a same input level with that for the APD together with a temperature sensor and a memory. The circuit provides the optimal bias voltage reading out from the memory depending on the input optical level, which is monitored by the PIN-PD, and the temperature monitored by the temperature sensor. Another Japanese Patent publication JP-H10-303820A has disclosed a configuration, in which the bias voltage is adjusted, when the input optical level instantaneously happens to vary in a low level by sensing this instantaneous fluctuation by the PIN-PD.
Still another Japanese Patent issued as JP-H01-029333B has disclosed a circuit that adjusts the current of the APD by installing a transistor connected in parallel to the APD and to the constant current source. This type of control, i.e., control of the current for the APD, shows a superior response to the type the bias voltage is adjusted. Japanese Patent publication as JP-H10-275129A has disclosed a light-receiving circuit that includes a transistor connected between the bias voltage source and the APD. By adjusting the base current of this transistor, the voltage drop between the collector and the emitter thereof may be adjusted, thus the bias voltage to the APD can be controlled.
Another Japanese Patent published as H05-343926 has disclosed an circuit for the APD, which provides a resistor connected in series to the APD and, by adjusting the resistance thereof and the bias voltage to the APD, the multiplication factor inherently attributed to the APD may be set in an optical value.
However, the method to apply the bias voltage to the APD using the parameter stored in the memory has an intrinsic disadvantage that the circuit can not follow the sudden change in the input optical level due to the memory read-out time, which causes an instantaneous increase of the APD current. Moreover, the method to reduce the bias voltage to the APD by the transistor or another active device connected between the bias voltage source and the APD, although showing the preferable response to the sudden change of the input optical level not only an addition circuit to control the active device is necessary but also the power dissipation due to this active device must be considered. Another circuit disclosed in the prior art, in which the transistor is connected in parallel to the APD, also consumes the surplus current to provide the optimal current to the APD.
In the circuit with the resistor for the current feedback to the APD, this resistor inevitably consumes the power, i.e., the resistor sets the optical bias condition for the APD by reducing the output of the voltage source, in particular, when the input optical level is large, the power dissipation due to this resistor becomes worse. Moreover, in order to secure a substantial multiplication actor of the APD, the output of the voltage source must be kept comparatively high, which further increases the power dissipation.
Therefore, an object of the present invention is to salve the subjects above mentioned and to provide a light-receiving circuit that enables to protect the APD, even when the APD receives the sudden optical pulse, by controlling the current flowing in the APD dynamically.